The present invention relates to manufacture of stainless steel products and has particular reference to manufacture of high performance torsion bars for motor vehicles.
The high performance features described below have great value for military vehicles such as tanks which are designed to be capable of maneuvering over large obstructions at increasingly higher speeds while still maintaining acceptable dynamics for the crew.
The desired properties of a high performance torsion bar are a specified torsional stiffness (angle of twist for a given length and twisting moment) and the highest possible cylic angle of twist consistent with survival of a specified number of applications of that twist.
To meet these requirements, the optimal material will have the lowest Young's modulus for its class and the highest cyclic elastic strain capability for the specified cyclic life. The elastic strain capability is usually enhanced by inducing a beneficial state of residual stress in the bar by twisting it beyond its yield point. This twisting leaves the bar ends permanently twisted under no torque. Although the larger the permanent twist the larger the beneficial effect, for each material there is a safe upper limit above which the material is damaged and no benefit accrues. With zero permanent twist, the allowable elastic strain increases with increasing cyclic strength.
It will be seen that among steel bars, the proposed procedure produces a steel with 10-20% reduction in Young's modulus with a high cyclic elastic strain capability because of its very high cyclic strength aided by an order of magnitude higher permanent twist than is possible with heat treated steels.
A number of metals display what is known as a martensitic transformation. The martensitic transformation is a rearrangement of the crystallographic structure without any change in the chemical composition of the crystal structure and results in a material characterized by new mechanical properties. The transformation is diffusionless. Moreover, such transformation in materials in which they occur are spontaneous at certain temperatures. For instance, as the temperature of the material is dropped, a temperature point will be reached where a martensitic transformation will commence occurring spontaneously. This temperature is known as the Ms temperature. The martensitic transformation will progress further as the temperature of the material continues to be dropped until at a certain temperature, generally known as the Mf temperature, there will be maximum spontaneous martensitic transformation, that is, as much martensitic as can be formed will be formed. It has been found that the martensitic transformation can be started above Ms temperature if the material is plastically deformed, that is, if irreversible mechanical work is put into the material. However, there is a maximum temperature above which no martensitic transformation will occur even if deformation takes place. This temperature is known as the Md temperature. Moreover, it has been found that at temperatures below the Ms temperature, the martensitic transformation can be made to progress further than it normally would spontaneously, provided the material is mechanically deformed at such a temperature.
In view of this knowledge and finding, we have discovered that with torsion springs made of materials which exhibit a martensitic transformation, it is desirable to deform the spring blank at a temperature below the Md temperature, and, preferably, close to the Ms temperature and most preferably at or slightly below the Ms temperature. When the deformation of such blanks is performed at such temperatures, the blank will gain in strength not only due to the inelastic stretching of the material but also due to the crystallographic transformation of the material to the generally stronger martensitic phase.
This invention exploits the known fact that austenitic stainless steels are capable of extremely high strains with a concurrent permanent increase in ultimate and yield strengths when worked at temperatures where martensitic transformation takes place, i.e. between the Md and Ms temperatures. For many of the popular grades of stainless steel the Md temperature is near that of dry ice (-100.degree. F) while the Ms temperature is very low and comparable to that of liquid nitrogen (-320.degree. F). However, the novelty of the invention lies in the particular sequence of operations it employs to strain harden the material at the martensitic transformation temperatures.
The essential steps for making a torsion spring according to the invention are these:
1. Determine stress-strain relationships in tension and in torsion at the temperature at which the working will take place, e.g. the Md temperature of the material to be used. Preferably specimens of the actual batch of raw material to be used will be so tested. Also, determine the change in density undergone by this material during martensitic transformation.
2. Using this data establish the required dimensions of the workpiece prior to straining such that the strained article will have the desired dimensions without subsequent machining.
3. Fabricate the torsion bar blank according to the dimensions determined in step 2. Preferably the heavier end regions are created by upsetting the ends of the bar. The end splines are made by forming or machining grooves into the upset ends prior to straining operations.
4. Place the blank into an environment which brings its temperature below the Md temperature thereof.
5. Torsion load the blank to a stress level between the yield point and ultimate. The direction of twist must correspond to that which the torsion bar will experience when installed and in operation.
For this procedure to be of value, the bar must be made to substantially final dimensions, including the spline region, taking into account the decrease in density which occurs during the martensitic transformation. Machining the spline region after forming would adversely affect this bar by removing portions of the strengthened structure. It should be recognized that the spline bearing region will yield and deform somewhat. The dent produced by a loose mating spline will be straight except for the effect produced by small variations in elastic springback along the length of each spline. The deepest region of the spline cavity where no bearing occurs may twist with respect to the straight dent made by the external spline tooth.
The extensive straining in torsion with subsequent metallurgical changes in the grain structure, produces a substantially higher final strength than that achievable by cold working procedures normally used to harden these stainless steels. The yield strength is also increased substantially, and most significantly, a piece fabricated by this process achieves a high endurance limit. Although strength or endurance of this magnitude is exhibited by other steels, it is the unique combination of low cost, flexibility, beneficial higher residual stress, high strength, and high endurance obtained by this process, combined with the inherent corrosion resistance of the material, which produces a superior torsion bar.
A particularly significant point of superiority exists at the splines, where stress concentrations are responsible for many premature fatigue failures in existing bars. The toughness and notch resistance of bars made by the new process, aided by large regions of high beneficial residual stresses in the spline reduce the likelihood of this type of failure.
In one alternative, the spline region could be worked by compression, twisting or bending to strengthen them prior to cutting the splines and final twisting.
In other alternatives, some refinements to the basic procedure may be added if desired. For example, prior to the torsion working, the workpiece may be worked in tension at a martensitic transformation temperature to strengthen the spline regions before forming them on the bar ends. The tension load applied should be about 80-95% of the ultimate.
Also, after torsion working, the article could be precipitation hardened, for example by aging in a heated oven and subsequent cooling to room temperature without quenching. For a material such as ASTI 304 or ASTI 301 the torsion bar would be heated for 20 hours at 780.degree.-790.degree. F. The value of precipitation hardening depends on the degree of cold work; for severe cold work, precipitation reduces the resulting strength.